1. Field of the Invention
The present invention relates to a multiple-shaft power transmission apparatus and a wafer transport arm link used in a processing apparatus for the manufacture of integrated circuits.
2. Description of Related Art
A wafer transport arm link is used to transport a wafer or other object to be processed between a process chamber and a transport chamber in a vacuum, a reduced pressure atmosphere, the air, or a corrosive gas.
The frogleg type of robot arm disclosed in Publication 1 (Japanese Unexamined Patent Publication No. 4-152078) was used at the initial stage of this technology. Also, a magnetic fluid has become the most common choice for a vacuum sealed shaft. Wafer transport arm links of this type are disclosed, for example, in Publication 2 (Japanese Unexamined Patent Publication No. 4-279043), Publication 3 (PC (WO) 7-504128), and Publication 4 (Japanese Unexamined Patent Publication No. 10-581).
Also, Publication 5 (PC (WO) 7-507010) discloses an example of a parallel link type of arm link that is different from the frogleg type. This link has a higher rigidity because it consists of four shafts and four arms.
One problem with the above-mentioned configuration lies in the constitution whereby the vacuum sealing force is generated from the viscous force and frictional force of a so-called magnetic fluid induced by magnetic force. With this constitution, a frictional force or viscous force proportional to the shaft rotation speed is generated as resistance during the operation of the robot arm. The drive force must therefore be large enough to overcome this resistance. Also, high-precision rotating shafts with a coaxial construction and polyaxial construction are required for the robot arm to perform rotational movement and extension and retraction movement. Thus, these rotating shafts are inevitably large, heavy, and complicated.
Furthermore, with a vacuum sealed shaft that makes use of a magnetic fluid, there is the danger that bubbles inside the magnetic fluid will expand and burst as the arm chamber is exhausted from atmospheric pressure to a vacuum, causing the magnetic fluid to scatter as microparticles and foul the vacuum chamber. This does not only lead to lower production yield for integrated circuits due to particle generation; because this magnetic fluid has a high iron content, it also causes heavy metal contamination of the integrated circuit.
As an alternative to this method, Publication 6 (Japanese Unexamined Patent Publication No. 3-136779) discloses an example of an arm shaft that makes use of a magnetic coupler disposed via a partition. The problems with the above-mentioned magnetic fluid type are solved with this approach, but because a powerful magnet is required to strengthen the coupling of the magnetic coupler, the robot ends up being large and heavy. When an arm shaft such as this is accelerated or decelerated, the movement of the arm is accompanied by vibration. This vibration is one of the factors that lead to dust generation during processing, shifting of the installation position, decreased throughput, and so on. Publication 6 discloses a constitution whereby vibration is prevented by generating an induced current at the partition. However, rather than trying to suppress the vibration of the arm, it would probably be more effective to design an arm that did not vibrate in the first place.
Publication 7 (Japanese Unexamined Patent Publication No. 7-245333) discloses a way to strengthen magnetic coupling. Publication 7 proposes that the magnetic binding force be increased by moving the permanent magnets that constitute the magnetic coupler as close together as possible. With this constitution, however, imparting the required positional precision to the arm link results in excessive stress being concentrated in the arm shafts. Also, since large-diameter wafers will be introduced in the near future as a means for lowering production costs, the arm transport distance will be even greater. This means that higher rigidity will be required of arm shafts.
In addition, arm shaft structures that utilize magnetic couplers are also disclosed in Publication 8 (Japanese Unexamined Patent Publication No. 61-69365) and Publication 9 (Japanese Unexamined Patent Publication No. 60-116960). Nevertheless, the above-mentioned problems are not solved by the structures disclosed in these publications.
Meanwhile, increasing the drive speed of a robot arm and thereby reducing the time it takes for wafer transport is important in terms of lowering unit production costs. However, the higher the drive speed is, the more particles are generated, the reason for which is discussed below.
The primary cause of this lies in the attitude control mechanism of the wafer tray. In the past, an attitude control mechanism that made use of a belt or a pulley was provided at the distal end of the arm link in order to keep the transport direction of the wafer tray the same as the extension and retraction direction of the arm. When this attitude control mechanism is provided, the number of arm shafts has to be increased from four to five. This increase in the number of arm shafts makes the arm more prone to vibration.
Also, a belt or pulley cannot transmit power without friction. Particles tend to be generated in a vacuum or a reduced pressure environment, and these particles are generated in large quantity every time the belt or pulley is operated. Various measures have been taken in the past to prevent this generation of dust, but these all result in a more complicated arm structure and decrease the rigidity of the arm.
Furthermore, as part of an effort to improve the productivity of semiconductor integration elements, there has been a need in recent years for two independently moving arm links to be used closer together in order to achieve an increase in the number of wafers processed per unit of time (throughput). When two robots are thus used close together, however, there is a new problem in that the particles generated from each arm adhere to the wafers of the other arm.
As a result of recent advances in the integration of semiconductor integrated circuits, the smallest line width of a circuit pattern is now approaching 0.15 micron. Accordingly, the diameter and number of particles generated in the manufacture and processing of integrated circuits now need to be reduced to class 20 or less, which is approximately one-fifth or less of the smallest line width. This increasing miniaturization is demanding that an extremely clean environment, in the broad sense, is required throughout the processing procedure. In terms of reduced pressure processing, this means that not only is the above-mentioned reduction in dust generation required, but a higher vacuum density and a lower out-gas environment must also be achieved.
Also, there is an increasing need for the diameter of wafers to be increased as part of the ongoing effort to reduce costs in the manufacturing process, as mentioned above. Accordingly, load-lock chambers, arm chambers, and various processing chambers will become larger, and the transport range of the robot arm will also become larger. Meanwhile, there is also a need to raise the error precision of the wafer placement position in order to increase the effective number of semiconductor elements that can be achieved with a single wafer. Therefore, an extremely high level of rigidity must be ensured for the arm shafts of a robot.
As described above, particles can greatly affect the production yield of integrated circuits in a semiconductor integrated circuit production process. Consequently, the reduction of particles is an important goal. Meanwhile, a robot arm capable of higher throughput is an equally important goal from the standpoint of making a massproduction plant more economical.